Cleaning with liquid gases

ABSTRACT

A method for cleaning objects in a pressure vessel with liquefied gases is provided. The gas liquefied under pressure is conducted into the pressure vessel. The temperature of the liquefied gas then, or beforehand, is lowered below the critical temperature of the gas and cleaning is performed at least primarily below the critical temperature and below the critical pressure of the gas. Carbon dioxide at temperatures between −20° C. and +20° C. is especially suitable. The mechanical interactions produced by the increased density and viscosity of the liquefied gas reinforce cleaning. Reduced solvent capacity does not influence the effectiveness of the cleaning. Textiles or components can therefore be cleaned at lower pressure and temperature.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for cleaning objects in pressurevessel with liquefied gases.

A method of this kind for cleaning workpieces, especially metal pipes,containing organic residues such as oils and greases is known frompatent application WO92/14558. In this application, liquefied gases suchas carbon dioxide are used as the cleaning fluid. The cleaning fluid isconducted into a pressure vessel loaded with the workpieces andcirculated therein by means of an impeller. After the cleaning processis complete, a portion of the fluid laden with organic residues isconducted out of the pressure vessel together with fresh cleaning fluidinto another pressure vessel. The surface tension of the remainder ofthe fluid loaded with impurities is reduced by a turbine, causing theimpurities to precipitate out. The cleaned workpieces are then removedfrom the empty pressure vessel, while additional workpieces can becleaned in the second pressure vessel.

In addition, a device for cleaning smaller workpieces that usesliquefied gases is known from German patent document DE-42 30 385. Inthe German patent document, a drum is mounted rotatably and/or pivotablyin a pressure-tight container and is connected to a drive device. Therotation of the drum sets both the cleaning fluid in the pressure-tightvessel and the objects present in the drum in turbulent motion, thusincreasing the cleaning effect. Chemical solvents, mechanical scouringagents, and added insert gases can increase the cleaning effect. Topermit quasi-continuous operation, at least one lock for loading andunloading the objects is provided on the pressure-tight vessel. Theselocks make it unnecessary to completely vent the pressure-tight vesselto load or remove the objects.

Liquid carbon dioxide at temperatures between 20° C. and approximately30° C. is used in the method in the two patent documents mentionedabove, with pressures corresponding to the vapor pressure values.

In addition, a method for cleaning textiles by means of liquefied orsupercritical carbon dioxide is known from WO 94/01613. In this patentdocument, the carbon dioxide is conducted into a pressure vessel loadedwith the textiles at temperatures between 20° C. and 100° C. andcorresponding pressures between 60 and 350 bars. After the textiles havebeen washed, displacement rinsing is performed using a second fluid. Thesecond fluid is again a compressed gas such as air or nitrogen.

A method for removing impurities from a substrate is known from WO90/06189. In this patent document, a substrate is brought into contactby means of a compressed gas with fluid-like density at critical orsupercritical pressure, with the phase of the gas being shifted betweenthe liquid state and the supercritical state by varying the temperatureof the gas in a series of stages between supercritical temperatures andsubcritical temperatures. This multistage temperature variation isdiscontinued by a change in the cohesion energy content of the gas inthe dense phase. An effort is therefore made to adjust the solvency ofthe compressed gas relative to an impurity in an effective manner.Mechanical interactions between the compressed gas and the impuritiesare not taken into account.

These known cleaning methods utilize the cleaning ability of compressedfluids, which increases drastically in the supercritical range.Supercritical carbon dioxide with a temperature above 31° C. atpressures above 73.7 bars has a density comparable to the liquid phaseand a good solvency that increases even further with an increase intemperature. Of course, the interactions of the fluid with the substanceto be dissolved (impurities) are important for the success of thecleaning.

In the practical application, these cleaning methods suffer from animportant disadvantage that lies in the use of high (supercritical)pressures, which in turn necessitates the use of expensive pressurevessels with high energy and equipment costs.

The goal of the present invention is therefore to develop a cleaningmethod using liquefied gases in which the same cleaning results can beachieved at lower cost.

This goal is achieved according to the invention by virtue of the factthat gas liquefied under pressure is conducted into the pressure vesselcontaining objects to be cleaned. The temperature of the liquefied gasis then (or even earlier) lowered below the critical temperature of thegas. Cleaning is conducted at least primarily below the criticaltemperature and below the critical temperature of the gas.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The figure is a schematic diagram of a pressure vessel having a drumtherein for use with the method according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Surprisingly, it has been found that the density and viscosity of gasesto be compressed to the liquid phase can be increased by lowering thetemperature sufficiently far below its critical temperature. Themechanical interactions between the cleaning fluid and the impuritiesguarantee cleaning success although the solvency of the liquefied gasunder these physical parameters is drastically reduced in comparison tothe previous method.

The increased mechanical interaction lies in a higher mass transport offluid to the surface to be cleaned because of the increased density andin the greater shearing forces between the fluid and the substratesurface because of the increased viscosity of the fluid. The viscosityincreases exponentially with the reciprocal of the temperature.

To remove organic residues for example, noble gases such as helium orargon, hydrocarbons such as methane, ethylene, propane, ethene, orpropene, as well as trifluoromethane, carbon dioxide, dinitrogenmonoxide, and sulfur hexafluoride are suitable as fluids for example.

For cleaning, the liquefied gas is loaded into the pressure vessel untilit fills a portion thereof. The liquefied gas is then in equilibriumwith its gaseous phase. After a certain cleaning time, the pressurevessel may be further filled with liquefied gas until it is exclusivelyin the liquid phase. Then, the density and viscosity of the liquid canbe increased even further when the pressure in the pressure vessel isincreased while keeping the temperature constant.

For known reasons, carbon dioxide is especially preferable and is usedat pressures from below 60 bars with an equilibrium of the liquid andgaseous phases at temperatures between −20° C. and +20° C. for thecleaning method according to the invention.

Unexpectedly, in this parameter range the mechanical interactions of theliquefied carbon dioxide with high density and viscosity overcome thereduced solvency.

During cleaning or at the beginning thereof, the pressure vessel can befilled completely with liquid carbon dioxide, with the temperature thenbeing kept constant at a value between −20° C. and +20° C., and thepressure being raised to a value above the corresponding value on thevapor pressure curve.

The mechanical interactions can be increased if the liquefied gas and/orthe object to be cleaned are circulated in the pressure vessel 10 (seethe Figure). This is accomplished in known fashion via an impeller or arotatable drum 12 in the pressure vessel 10.

If the pressure vessel is only partially filled with liquefied gas,additional frictional action on the surface of the contaminated objectstakes place when the objects to be cleaned are circulated, as a resultof the objects being constantly lifted out of, and submerged in, theliquid phase.

When a rotating drum is used, the mechanical interaction can beincreased if the drum is operated intermittently and/or with a change inthe direction of rotation (see arrows in the Figure).

Especially good cleaning results for contamination with organic residuessuch as oils and greases are obtained by using carbon dioxide attemperatures of 5° C. to 15° C., preferably 10° C.

In this type of cleaning, pressure values that are lower by comparisonwith the known method are especially advantageous, beginning with atemperature drop in contrast to the temperature increase required inearlier cleaning methods. This means firstly a reduced energyexpenditure and secondly, lower system costs for system components thatare resistant to high pressure. For the cleaning method according to theinvention for example, liquefied carbon dioxide which is liquid at roomtemperature (25° C., 67 bars) is cooled to 10° C. and placed in apressure vessel designed for approximately 100 bars. The addition ofenzymes, emulsifiers, and/or surfactants (detergents) that are suitablefor liquefied carbon dioxide for example can further increase thesuccess of cleaning. Suitable additives will be found by the individualskilled in the art in the pertinent literature, for example in thespecification of European Patent document EP-0 530 949-A1.

The method according to the invention has proven in many tests to beespecially effective for cleaning textiles. It is also suitable forcleaning metal surfaces or electronic assemblies such as PC boards toremove impurities that mostly contain organic residues.

In one version of the method according to the invention, test fabriccontaminated with various greases was cleaned in a pressure vessel towhich liquefied carbon dioxide had been added. The pressure vesselcontains a drum that rotates inside the pressure vessel, causing thetextiles and the liquefied gas to move relative to one another. The gasis drawn from a supply container in which carbon dioxide liquefied underpressure is at ambient temperature, and the pressure vessel is partiallyfilled. The temperature in the pressure vessel is lowered to about 10°C., while the drum is set rotating.

During cleaning, the liquid and gaseous phases through which the testfabrics are transported mix with one another, so that frictional effectsoccur that promote cleaning. If necessary, after a certain period ofcleaning, the pressure vessel can be filled completely once again, andthe temperature of the liquid carbon dioxide is kept constant at about10° C. while the pressure is raised to more than 45 bars. The pressurecan be increased up to 70 bars (below critical pressure), for example.Cleaning results however are completely sufficient with a lesserpressure increase, so that operating and system costs can be reducedsignificantly in comparison to previous cleaning methods.

The working of the method and possible addition of surfactants can beselected as a function of the type of contamination. After cleaning, thecontaminated carbon dioxide is removed and can be reused after reductionof surface tension, when the impurities precipitate out. Thiscontributes to environmental protection and reduces costs further.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample, and is not to be taken by way of limitation. The spirit andscope of the present invention are to be limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method for cleaning objects in a pressurevessel using carbon dioxide, the method comprisingthe steps of :conducting the carbon dioxide under pressure into the pressure vesselcontaining the objects to be cleaned, wherein the pressure vessel isonly partially filled with liquid carbon dioxide; maintaining orlowering the temperature of the carbon dioxide below the criticaltemperature; and performing a cleaning operation at least primarilybelow the critical temperature and below the critical pressure of thecarbon dioxide, wherein liquid and gas phases of the carbon dioxide arein equilibrium for a portion of the cleaning, ; wherein the temperatureis kept constant at a value between −20° C. and +20° C. and the pressureis raised to a value above the corresponding vapor pressure during aportion of the cleaning.
 2. The method according to claim 1, wherein thecleaning operation further comprises the step of circulating at leastone of the carbon dioxide and the objects to be cleaned in the pressurevessel.
 3. The method according to claim 2, wherein said circulatingstep is carried out via a drum mounted at least one of rotatably andpivotably in the pressure vessel, and wherein said drum is operated atleast one of intermittently and with a changing direction of rotation.4. The method according to claim 1, wherein the carbon dioxide in thecleaning step is at a temperature of 5° C. to 15° C.
 5. The methodaccording to claim 1, wherein the carbon dioxide in the cleaning step isat a temperature of 15° C.
 6. The method according to claim 1, furthercomprising the step of adding at least one of enzymes, emulsifiers,surfactants, and detergents to the carbon dioxide.
 7. The methodaccording to claim 1, wherein said method is used for cleaning textiles.8. The method according to claim 1, wherein said method is used forcleaning components.
 9. The method according to claim 1, wherein thepressure vessel is only partially filled with the carbon dioxide. 10.The method according to claim 1, wherein the pressure vessel iscompletely filled with the carbon dioxide.
 11. The method according toclaim 1, wherein the pressure of the carbon dioxide is below 60 bars.12. The method according to claim 2, wherein the carbon dioxide in thecleaning step is at a temperature of 5° C. to 15° C.
 13. The methodaccording to claim 4, wherein the carbon dioxide in the cleaning step isat a temperature of 10° C.
 14. The method according to claim 2, furthercomprising the step of adding at least one of enzymes, emulsifiers,surfactants, and detergents to the carbon dioxide.
 15. The methodaccording to claim 4, further comprising the step of adding at least oneof enzymes, emulsifiers, surfactants, and detergents to the carbondioxide.
 16. The method according to claim 1, wherein the pressure ofthe carbon dioxide is between 45 and 70 bars.
 17. The method accordingto claim 4, wherein the pressure of the carbon dioxide is between 45 and70 bars.
 18. The method according to claim 12, wherein the pressure ofthe carbon dioxide is between 45 and 70 bars.
 19. The method accordingto claim 15, wherein the pressure of the carbon dioxide is between 45and 70 bars.
 20. The method according to claim 1, wherein the pressureof the carbon dioxide is above 45 bars.
 21. The method according toclaim 4, wherein the pressure of the carbon dioxide is above 45 bars.22. A method according to claim 1, wherein the temperature of the carbondioxide is maintained below the critical temperature.
 23. A method forcleaning objects in a pressure vessel using carbon dioxide, the methodcomprising: conducting the carbon dioxide under pressure into thepressure vessel containing the objects to be cleaned, wherein thepressure vessel is only partially filled with liquid carbon dioxide;maintaining or lowering the temperature of the carbon dioxide below thecritical temperature; adding enzymes to the carbon dioxide; andperforming a cleaning operation at least primarily below the criticaltemperature and below the critical pressure of the carbon dioxide,wherein liquid and gas phases of the carbon dioxide are in equilibriumfor a portion of the cleaning; wherein the temperature is kept constantat a value between −20° C. and +20° C. and the pressure is raised to avalue above the corresponding vapor pressure during a portion of thecleaning.
 24. A method for cleaning objects in a pressure vessel usingcarbon dioxide, the method comprising: conducting the carbon dioxideunder pressure into the pressure vessel containing at least one of anelectronic assembly or an object having a metal surface, wherein thepressure vessel is only partially filled with liquid carbon dioxide;maintaining or lowering the temperature of the carbon dioxide below thecritical temperature; and performing a cleaning operation at leastprimarily below the critical temperature and below the critical pressureof the carbon dioxide, wherein liquid and gas phases of the carbondioxide are in equilibrium for a portion of the cleaning; wherein thetemperature is kept constant at a value between −20° C. and +20° C. andthe pressure is raised to a value above the corresponding vapor pressureduring a portion of the cleaning.